This application is directed toward the study of visual mechanisms in observers with normal vision, observers with congenital color vision defects, and observers with acquired visual defects secondary to various disease states. The studies are designed to reveal the biological processes that limit human visual performance. The proposed research is organized into 6 projects: Project 1 concerns the mechanisms limiting the human observer's sensitivity at low light levels. Project 2 looks at the function of the two major processing streams between eye and brain, and how the parvocellular (PC) and magnocellular (MC) pathways are involved in achromatic discriminations. Measures will be taken for a variety of spatial tasks in normal observers, and PC and MC function will be measured in color defective observers and in patients with ophthalmic disease. Project 3 involves delineating the postreceptoral pathways for rod signals and rod-cone interactions. The proposed experiments examine the role of the rod system in daylight vision. Project 4 addresses the function of a newly discovered retinal ganglion cell type with the properties of an S-cone off cell. Experiments are designed to assess psychophysically the function of this new cell type. Project 5 describes continuing collaborative neurobiological studies of retinal function that allow the opportunity to compare retinal and psychophysical responses to the similar or identical light stimuli. Project 6 involves the development and evaluation of new clinical tests of visual function that may improve the functional diagnosis of congenital color vision defects and help differentiate patients with eye disease. The goal is to translate difficult and time-consuming psychophysical experiments into rapid and easily performed clinical tests. In addition to empirical studies, models of normal and abnormal vision will continue to be developed. The theoretical approach is based on the question: to what extent can established biological or physical phenomena explain the data of color vision? This approach is one which allows us to examine the plausibility of various hypotheses that are not subject to direct experimental evaluation.